Transform-based image coding method and apparatus therefor

Abstract

An image decoding method according to the present document comprises the steps of: on the basis that an intra prediction mode of a chroma block is a cross-component linear model (CCLM) mode, updating the intra prediction mode of the chroma block on the basis of an intra prediction mode of a luma block corresponding to the chroma block; and determining an LFNST set including LFNST matrices on the basis of the updated intra prediction mode, wherein the updated intra prediction mode is derived as an intra prediction mode corresponding to a specific location in the luma block, and on the basis that the intra prediction mode corresponding to the specific location is an MIP mode, the updated intra prediction mode is updated to an intra planner mode.

What is claimed is: 
     
         1 . An image decoding method performed by a decoding apparatus, the method comprising:
 obtaining intra prediction mode information from a bitstream;   updating an intra prediction mode of a chroma block based on an intra prediction mode of a luma block corresponding to the chroma block, based on the intra prediction mode of the chroma block being a cross-component linear model (CCLM) mode;   determining an LFNST set comprising LFNST matrices based on the updated intra prediction mode; and   deriving transform coefficients for the chroma block based on the LFNST matrix derived from the LFNST set; and   deriving residual samples for the chroma block based on the transform coefficients,   wherein the updated intra prediction mode is derived as an intra prediction mode corresponding to a specific position in the luma block, and   wherein based on an intra prediction type corresponding to the specific position being an MIP mode, the intra prediction mode of the chroma block is updated to an intra planar mode.   
     
     
         2 . The image decoding method of  claim 1 , wherein the specific position is set based on a color format of the chroma block. 
     
     
         3 . The image decoding method of  claim 2 , wherein the specific position is a center position of the luma block. 
     
     
         4 . The image decoding method of  claim 3 , wherein the specific position is set to ((xTbY+(nTbW*SubWidthC)/2), (yTbY+(nTbH*SubHeightC)/2)),
 wherein xTbY and yTbY denote top-left coordinates of the luma block,   wherein nTbW and nTbH denote a width and a height of the chroma block, and   wherein SubWidthC and SubHeightC denote variables corresponding to the color format.   
     
     
         5 . The image decoding method of  claim 4 , wherein when the color format is 4:2:0, SubWidthC and SubHeightC are 2, and
 wherein when the color format is 4:2:2, SubWidthC is 2 and SubHeightC is 1.   
     
     
         6 . The image decoding method of  claim 1 , wherein when a prediction mode corresponding to the specific position is an IBC mode, the intra prediction mode of the chroma block is updated to an intra DC mode. 
     
     
         7 . The image decoding method of  claim 1 , wherein when a prediction mode corresponding to the specific position is a palette mode, the intra prediction mode of the chroma block is updated to an intra DC mode. 
     
     
         8 . An image encoding method performed by an image encoding apparatus, the method comprising:
 deriving prediction samples for a chroma block based on an intra prediction mode for the chroma block being a cross-component linear model (CCLM);   deriving residual samples for the chroma block based on the prediction samples;   updating the intra prediction mode of the chroma block based on an intra prediction mode of a luma block corresponding to the chroma block;   determining an LFNST set comprising LFNST matrices based on the updated intra prediction mode; and   deriving modified transform coefficients for the chroma block based on the residual samples and the LFNST matrix,   wherein the updated intra prediction mode is derived as an intra prediction mode corresponding to a specific position in the luma block, and   wherein based on an intra prediction type corresponding to the specific position being an MIP mode, the intra prediction mode of the chroma block is updated to an intra planar mode.   
     
     
         9 . The image encoding method of  claim 8 , wherein the specific position is set based on a color format of the chroma block. 
     
     
         10 . The image encoding method of  claim 9 , wherein the specific position is a center position of the luma block. 
     
     
         11 . The image encoding method of  claim 10 , wherein the specific position is set to ((xTbY+(nTbW*SubWidthC)/2), (yTbY+(nTbH*SubHeightC)/2)),
 wherein xTbY and yTbY denote top-left coordinates of the luma block,   wherein nTbW and nTbH denote a width and a height of the chroma block, and   wherein SubWidthC and SubHeightC denote variables corresponding to the color format.   
     
     
         12 . The image encoding method of  claim 11 , wherein when the color format is 4:2:0, SubWidthC and SubHeightC are 2, and
 wherein when the color format is 4:2:2, SubWidthC is 2 and SubHeightC is 1.   
     
     
         13 . The image encoding method of  claim 8 , wherein when a prediction mode corresponding to the specific position is an IBC mode, the intra prediction mode of the chroma block is updated to an intra DC mode. 
     
     
         14 . The image encoding method of  claim 8 , wherein when a prediction mode corresponding to the specific position is a palette mode, the intra prediction mode of the chroma block is updated to an intra DC mode. 
     
     
         15 . A non-transitory computer-readable digital storage medium that stores a bitstream generated by a method, the method comprising:
 deriving prediction samples for a chroma block based on an intra prediction mode for the chroma block being a cross-component linear model (CCLM);   deriving residual samples for the chroma block based on the prediction samples;   updating the intra prediction mode of the chroma block based on an intra prediction mode of a luma block corresponding to the chroma block;   determining an LFNST set comprising LFNST matrices based on the updated intra prediction mode;   deriving modified transform coefficients for the chroma block based on the residual samples and the LFNST matrix; and   encoding residual information to generate the bitstream,   wherein the updated intra prediction mode is derived as an intra prediction mode corresponding to a specific position in the luma block, and   wherein based on an intra prediction type corresponding to the specific position being an MIP mode, the intra prediction mode of the chroma block is updated to an intra planar mode.